What Causes Short Shot in Injection Molding?

1. What is a Short Shot?

A short shot is a term commonly used in plastic injection molding, which refers to a situation where the mold cavity is not completely filled with molten plastic material during the injection molding process.

In this situation, the plastic part that is produced is incomplete, with some areas missing material, resulting in a part with short shots or voids. This can occur due to a variety of reasons, such as insufficient injection pressure, inadequate melt temperature, or improper gating or venting.

2. Causes of Short Shots

2.1. Improper Selection of Equipment

When selecting equipment, the maximum injection volume of the injection molding machine must be greater than the total weight of the plastic part and nozzle, and the total injection weight cannot exceed 85% of the plasticized volume of the injection molding machine.

2.2. Insufficient Material Supply

One of the primary causes of short shots is an insufficient supply of material entering the mold cavity. This can happen if the material hopper runs out of plastic or if the feeding system malfunctions. A blocked or worn-out nozzle can also restrict material flow, preventing the cavity from filling completely.

2.3. Incorrect Material Viscosity

Material viscosity, or the resistance to flow, plays a crucial role in the injection molding process. If the viscosity is too high, the material may not flow smoothly into the mold cavity, causing a short shot. Factors such as temperature, material type, and additives can influence viscosity.

2.4. Insufficient Injection Pressure or Holding Pressure

The injection pressure is close to a proportional relationship with the filling length. If the injection pressure is too small, the filling length is short and the cavity is not filled enough. In this regard, the injection pressure can be increased by slowing down the injection speed and prolonging the injection time appropriately.

2.5. Unreasonable Gating System Design

When a mold has multiple cavities, the appearance defects of the plastic part are often caused by the unreasonable balance design of the gate and the runner. When designing the gating system, pay attention to the balance of the gate. The weight of the plastic parts in each cavity should be proportional to the size of the gate, so that each cavity can be filled at the same time. A balanced layout designs.

2.6. Inadequate Mold Venting

When a large amount of gas remaining in the mold due to poor exhaust is squeezed by the flow material and generates a high pressure greater than the injection pressure, it will prevent the melt from filling the cavity and cause short shots.

2.7. Low Mold Temperature

After the molten material enters the low-temperature mold cavity, it will not be able to fill all corners of the cavity due to cooling too fast. Therefore, the mold must be preheated to the temperature required by the process before starting the machine. When the machine is first started, the cooling water flow in the mold should be properly controlled. If the mold temperature does not rise, check whether the design of the mold cooling system is reasonable.

2.8. Unreasonable Part Structural Design

When the thickness of the plastic part is not proportional to the length, the shape is very complex and the molding area is large, the flow of molten material is easily blocked at the entrance of the thin-walled part of the plastic part, making it difficult to fill the cavity. Therefore, when designing the shape and structure of plastic parts, it should be noted that the thickness of the plastic part is related to the limit flow length when the molten material is filled.

2.9. Low Melt Temperature

Usually, within the range suitable for molding, the material temperature and the filling length are close to a proportional relationship, and the flow performance of the low-temperature melt decreases, which shortens the filling length. When the material temperature is lower than the temperature required by the process, check whether the barrel feeder is intact and try to increase the barrel temperature.

3. How to Prevent Short Shots

To prevent short shots, consider the following recommendations:

3.1. Material selection and preparation

Ensure that you're using the appropriate material for your project, and that the material is thoroughly dried before use. Moisture in the material can cause short shots and other molding defects.

3.2. Mold design

Check the mold design to ensure proper gating, venting, and cooling. Complex or long flow paths may require modifications, such as adding more gates, to achieve complete filling.

3.3. Injection pressure

Increase the injection pressure to ensure that the material flows smoothly and fills the mold cavity. Keep in mind that excessively high pressure can cause other issues, such as flash or material degradation.

3.4. Injection speed

Adjust the injection speed to find a balance between filling the mold quickly and avoiding excessive shear stress on the material. Typically, a faster injection speed is more likely to prevent short shots, but the optimal speed will depend on the specific molding conditions.

3.5. Melt temperature

Ensure that the material is heated to the correct temperature, as specified by the material manufacturer. Inadequate melt temperature can cause the material to cool prematurely, resulting in short shots.

3.6. Mold temperature

Adjust the mold temperature to prevent premature cooling of the material. The ideal mold temperature will depend on the specific material and part geometry, but generally, higher mold temperatures can help avoid short shots.

3.7. Hold pressure and time

Optimize the hold pressure and hold time to ensure that the material has enough time to fill the mold and compensate for shrinkage during cooling. Hold pressure should be high enough to maintain the cavity pressure but not so high that it causes part defects.

3.8. Clamping force

Ensure that the clamping force is sufficient to keep the mold closed during the injection process. Insufficient clamping force can cause the mold to open slightly, leading to short shots and other defects.

3.9. Cycle time

Optimize the cycle time to ensure that the material has enough time to fill the mold and cool properly. Reducing cycle time too much may lead to short shots, while excessively long cycle times can reduce productivity.

3.10. Regular maintenance

Perform regular maintenance on the injection molding machine and mold to ensure proper functioning and minimize the risk of short shots and other defects.

4. Conclusion

Short shots in injection molding can lead to production delays and increased costs. By understanding the common causes of short shots and implementing the appropriate preventative measures, manufacturers can maintain production efficiency and minimize defects. Regular monitoring, mold design optimization, and proper process control are essential for preventing short shots and ensuring the consistent production of high-quality plastic parts.